1. Field of the Invention
The present invention relates to a semiconductor device having a circuit that comprises a thin film transistor (hereinafter referred to as TFT) and to a method of manufacturing the same. For example, the invention relates to a display device, such as a liquid crystal display device (with a liquid crystal module mounted thereto) and to electronic equipment having the display device as its component.
In this specification, the term semiconductor device refers to devices in general that utilize semiconductor characteristics to function, and electro-optical devices, semiconductor circuits, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
In recent years, a technique of constructing a thin film transistor (TFT) from a semiconductor thin film (thickness: about several nm to several hundreds of nm) formed on a substrate having an insulating surface is drawing attention. Thin film transistors are applied to a wide variety of electronic devices including ICs and electro-optical devices, and rapid development of thin film transistors as switching elements for image display devices is demanded.
Liquid crystal display devices are one of conventionally known image display devices. Active matrix liquid crystal display devices are now employed more often since they can display images with higher definition as compared to passive matrix liquid crystal display devices. In an active matrix liquid crystal display device, pixel electrodes arranged so as to form a matrix are driven to form a display pattern on a screen. Specifically, a voltage is applied between a selected pixel electrode and an opposite electrode associated with the selected pixel electrode to subject a liquid crystal layer placed between the pixel electrode and the opposite electrode to optical modulation. This optical modulation is recognized as a display pattern by an observer.
Uses of active matrix liquid crystal display devices are increasing and they are demanded to be larger in screen size and higher in definition, aperture ratio, and reliability. Also requested are improved productivity and lowered cost.
When aluminum is used as a gate wiring line material in manufacturing a TFT, prior art cannot avoid operation failure of the TFT and degraded TFT characteristics due to heat treatment which causes projections such as hillock or whisker and diffusion of aluminum atoms to a channel formation region. As a countermeasure, a metal material that can withstand the heat treatment, typically a metal element having a high melting point, is used for the gate wiring line. In this case, however, such problems occur that the wiring line resistance is raised as the screen becomes larger and the like, thereby increasing power consumption, etc.